The present invention relates to an apparatus and method for positioning a dye donor web in a thermal color printer for minimal waste and cost.
In a color thermal printing process of a thermal color printer, the finished print is made by successively transferring color dyes from respective dye patches of a dye donor web onto a dye receiver medium with a thermal printer head. In general, a dye donor web contains a repeating series of frames of different colored, heat transferable dyes. The color series of a dye donor web can be YMC (yellow, magenta, cyan), YMCK (YMC, black), YMCO (YMC, overlay) or YMCKO. During the color printing process, it is necessary to have the dye donor web properly positioned relative to the dye receiver medium to ensure full coverage of the image area by successive color frames. Since the donor web has a repeating series of different colored dye frames, it is necessary to identify the leading edge of each different frame of each color series. One way to do this is to provide sensor marks on the donor, such as disclosed in U.S. Pat. No. 5,466,075 which inserted different black bars at the leading edges of the colored frames (the yellow frame) of each series as the sensor marks. Another way to identify the frames is to provide color discriminating optical sensors directly located in the donor web path just past the print line of the thermal print head in the direction of travel of the donor web, such as disclosed in U.S. Pat. Nos. 4,710,781 and 5,266,967. These sensors detect the presence of different colored patches on the donor as they move forward. The particular dye frame can be identified by analyzing the light intensity of transmission or reflection of a light beam or a plurality of light beams with different colors. It is noted that the sensor marks and the physical configuration of the print head and surrounding mechanisms indeed limit the minimum size of the color frames. Among the consequences of having unused donor are: a higher cost of material for making prints, reduced donor web capacity in the printer, and a great amount of material requiring environmentally safe disposal after use.
As shown in FIG. 1, the thermal color printer of the prior art comprises a dye donor web 10 having a plurality of dye frames in a repeating series of different colors; a web drive 20 adapted to move and receive the dye donor web 10, which includes a ribbon supply spool 21, a received spool 22, a motor 23 used to rotate the received spool 22, two idle rollers 24, 25 and a platen roller 26; a sensor device 30 adapted between the idle roller 25 and the platen roller 26, which consists of a light source 31 and an optical sensor 32 for identifying the dye frames; and a controller 40 used to position the dye donor web 10 with high precision by properly driving the web drive 20. During the color thermal printing process, the web drive 20 is driven by the controller 40 to deliver the dye donor web 10 adapted on the ribbon supply spool 21 to the received spool 22 through the idle roller 24, the platen roller 26 and the idle roller 25 for taking up the used donor web. At the opposite position of the platen roller 26, there is a thermal print head 50, and a print line 60 is consequently formed between the thermal print head 50 and the platen roller 26. The thermal print head 50 is energized to transfer dye from the donor web at the print line 60 to a receiver medium. In the system shown in FIG. 1, it is desirable to position the sensor device 30 as close as possible to the print line 60 because the amount of the donor web 10 after positioning is not used in printing, and is therefore wasted. Unfortunately, the physical configuration of the print head and surrounding mechanisms indeed limit the minimum distance that can be achieved. This, in sum, limits the minimum size of the color frames. Among the consequences of having unused donor are: a higher cost of material for making prints, reduced printing capacity in the printer, and increased volume of material requiring environmentally safe disposal after use.
In order to minimize the waste of the donor web, an improved configuration of the thermal color printer was disclosed in U.S. Pat. No. 4,710,781 and shown as FIG. 2. A motor 27 linked with the ribbon supply spool 21 is programmed to rotate the ribbon supply spool 21 through a predetermined arc length in the reverse-feed direction to draw the web 10 backward for minimizing the distance between the leading edge of every color frame and the print line 60. However, since the distance of web moving is not only a function of the amount of supply spool rotation, but also a function of supply roll diameter, the amount of rotation of the ribbon supply spool 21 must be determined for a full supply roll. Thus, operation with anything other than a full supply roll still results in additional dye donor web waste. In U.S. Pat. No. 5,549,400, an encoder wheel 28 and an encoder sensor 29 are disposed in the ribbon supply spool 21 for positioning the dye donor web 10 with a high precision control to the rotation angle of the ribbon supply spool 21. This configuration indeed reduces the waste of the web, however increases the cost and assembly complexity of the thermal color printer.
The drawbacks of the thermal color printers of the prior art are listed as follows:
1. In order to ensure full coverage of the image area by successive color frames, the optical sensors directly located in the donor web path just past the print line of the thermal print head in the direction of travel of the donor web. The amount of dye donor web between the sensors and the print line of the thermal print head is unused.
2. In order to identify the color frame, sensor marks are inserted on the donor web. The total length of the donor web is consequently reduced and may result in reducing the capacity of printing.
3. In the configuration of disposing a motor in the supply spool, since the rotation angle of the motor is a function of the spool roll diameter, the operation with anything other than a full supply roll still results in additional dye donor web waste.
4. The cost and assembly complexity of the thermal color printer with additional encode wheel and sensors are obviously increased.